We have demonstrated that the Type II IGF receptor is phosphorylated in intact cells. The phosphorylation is not dependent upon IGF-II and could not be demonstrated with solubilized receptor preparations. We have developed two assays to screen mouse sera and hybridoma supernatant for antibodies to the type II IGF receptor. One assay measures the ability of the serum or supernatant to block the binding of radiolabeled IGF-II to type II receptor bearing cells (blocking antibody). The second assay is an immunoprecipitation assay which measures the ability of the serum or hybridoma supernatant to bind to a preforemed radioligant-solubilized receptor complex and be immunoprecipitated with a goat antimouse serum. Using these assays we have measured receptor antibodies in sera of mice whose spleen cells are being fused to plasmacytoma cells (NS-1) to form hybridomas. We have shown that mouse embryonal carcinoma cell lines produce IGF-II but little or not IGF-I. We are characterizing IGFs and IGF binding proteins produced by human fetal fibroblasts and postnatal fibroblasts.